Ammonia is a widely used chemical with many applications. Specific applications include using ammonia as reductant for selective catalytic reduction (SCR) of NOx in exhaust gas from combustion processes or using ammonia as fuel in energy generating processes as for example in relation to fuel cells.
For most applications, and in particular in automotive applications, the storage of ammonia in the form of a pressurized liquid in a vessel is too hazardous. Urea is a safe, but an indirect and impractical method for mobile transport of ammonia since it requires urea to be transformed into ammonia by a process involving thermolysis and hydrolysis ((NH2)2CO+H2O→2NH3+CO2).
A storage method involving ad- or absorption in a solid can circumvent the safety hazard of anhydrous liquid ammonia and the decomposition of a starting material.
Metal ammine salts are ammonia absorbing and desorbing materials, which can be used as solid storage media for ammonia (see, e.g. WO 2006/012903 A2), which in turn, as mentioned above, may be used as the reductant in selective catalytic reduction to reduce NOx emissions.
Ammonia release from the ammonia storage materials is an endothermic process that requires supply of heat. An associated problem is that the storage materials and especially the ammonia depleted storage materials in general have low thermal conductivity and upon depleting ammonia from the material porosities may form, which inhibit heat conduction even more. The effects of deteriorating heat conduction are that the heating source has to be heated to higher temperature and the response time of the system becomes longer.
Another problem arises from material properties changing upon depletion of ammonia from the ammonia storage materials. Because ammonia is a substantial part of the structure of the materials, most ammonia-absorbing solids shrink in overall dimensions upon depletion. If the material initially completely fills a container, it will loose contact with the container walls after degassing. The gap between the container wall and the storage material will act as an insulation layer and prevent heat being transported into the storage material if the container is heated from the outside. It is also undesirable to have a large block of material loosely contained in a container which is mounted on a moving and vibrating vehicle, as this may compromise the mechanical stability of the system.
The present invention addresses these problems.